Fig. 5.

Polyoxometalate (POM)-5 is a potent, noncompetitive, reversible inhibitor of ATP hydrolysis in human bronchial epithelial cells. A: hydrolysis of [γ³²P]ATP was assessed for 15 min in the presence of the indicated concentration of POM-1, POM-4, POM-5, or POM-6. B: the initial rates of hydrolysis of 30, 100, 300, and 1,000 μM ATP were measured in the absence or presence of 30 μM POM-5. The data in A and B represent the mean ± SD from two experiments performed with duplicate samples. The sigmoidal curves (A) and exponential rise regression lines (B) were fitted using Sigma Plot 10. C: inhibition of ATP hydrolysis is greatly reversed upon removal of POM-5: open squares, cells were preincubated (30 min) with POM-5 and then washed and incubated with [³²P]ATP in the absence of POM-5 for 15 min; closed squares, naïve cells were incubated with [³²P]ATP in the presence of 30 μM POM-5. The data are expressed as % ATP hydrolysis relative to cells that were not exposed to POM-5. Values (mean ± SD; n = 4 per condition) are representative of three independent experiments. *P < 0.05; **P < 0.001.